1. Field of the Invention
The present invention relates to a method and an apparatus for measurement of a liquid level using a heat radiating type level sensor, suitable for the detection of the fuel level in a fuel tank of an automobile.
2. Description of the Background Art
A heat radiating type level sensor is a sensor for measuring the liquid level which utilizes the fact that the resistivity of the sensor made of a resistive body changes according to a depth by which the sensor is immersed into the liquid. As an example, a method using a pulse shaped constant current will now be described.
In this method, a constant current is conducted through the sensor for several seconds, such that the sensor output voltage is increased, where the amount of increase of the sensor output voltage corresponds to the liquid level. Here, however, the voltage level difference between FULL state and EMPTY state is small at the end of the current conduction period, so that the practically sufficient resolution cannot be achieved by the straightforward measurement of the increased voltage level. For this reason, in this method, the output proportional to the liquid level is obtained from an average slope for the increase of the sensor output voltages instead of the amount of increase of the sensor output voltages.
More specifically, the sensor output voltage is digitally recorded in every few msec, and the slope is derived by the linear approximation processing carried out at a micro-computer, so as to improve the resolution in the output, as follows.
Namely, as shown in a graph of FIG. 1B in which a horizontal axis represents time and a vertical axis represents current, a current conduction starts at a time t.sub.0, and a constant current is applied between a time t.sub.0 ' and a time t.sub.f and such a pulse shaped current conduction is repeated intermittently. In response to this pulse shaped current conduction, as shown in a graph of FIG. 1A, the sensor output voltages V.sub.1, V.sub.2,--, V.sub.f are measured at time t.sub.1, t.sub.2,--, t.sub.f, respectively. Then, each of the measured sensor output voltages is divided by the initial sensor output voltage V.sub.1 in order to compensate the environmental temperature dependence. Next, as shown in FIG. 2, a linearly approximated output voltage Vtc' at a time tc is obtained by linearly approximating the slope of the increase of the measured output voltages with respect to time using the sensor output voltages measured at time t.sub.1 to t.sub.n. Then, as shown in FIG. 2, according to this linearly approximated output voltage Vtc', the steady state output voltage Vtc at a time tc is derived as a measured liquid level by using the appropriate data processing.
Now, as shown in FIG. 3A, the measured output voltage changes in response to the pulse shaped current conduction shown in FIG. 3B, where a slope Sa is a case of the high enviromental temperature and a slope Sb is a case of the low environmental temperature. As can be seen in FIG. 3A, when the sensor temperature is considered to be identical to the environmental temperature at the beginning of the current conduction period, the the measured output voltage becomes larger for the higher environmental temperature.
For this reason, as shown in FIG. 4A, the environmental temperature dependence of the measured output voltages is compensated in a slope Sc corresponding to the pulse shaped current conduction shown in FIG. 4B, which is obtained by dividing the measured output voltages by the initial measured output voltage V.sub.1. This slope Sc is almost identical in either case of the high or low environmental temperature.
However, as shown in FIG. 5A, by comparing a slope Sd for the measured output voltages in a case in which the sensor temperature is equal to the environmental temperature and a slope Se for the measured output voltages in a case in which the sensor temperature is lower than the environmental temperature, both of which are obtained in response to the pulse shaped current conduction shown in FIG. 5B and subjected to the above described environmental temperature dependence compensation, it can be seen that even when the initial output voltage is the same, the slope Se can be smaller than the slope Sd.
In other words, the above described environmental temperature dependence compensation assumes that the sensor temperature is identical to the environmental temperature. Consequently, when there is a sudden change in the environmental temperature for example, the sensor temperature cannot follow this sudden change of the environmental temperature because of the large heat capacity of the sensor, such that the measured output voltages can be lowered as in the slope Se shown in FIG. 5A, and the errors can be introduced into the measured output voltages.
In addition, in the application to the fuel tank of an automobile, there is a case in which the fuel is agitated to change the fuel level during the running of the automobile, such that the correct remaining fuel level cannot be obtained.
Furthermore, in the heat radiating type level sensor, the sensor is heated during the measurement, so that the sensor temperature is higher than the environmental temperature after each measurement, so that there is a need to provide a cooling period for cooling down the sensor temperature to the level of the environmental temperature, between each successive measurements. Consequently, the measurement interval becomes considerably long, and this in turn prevents the use of a simple data processing such as the averaging of a large number of measurements made in a short period of time.